Polymers comprised of polyimides have been found to have a variety of desirable qualities that are useful in high temperature applications. In general, and particularly for use as matrix materials in structural composites, the desirable qualities include a high glass transition temperature (Tg), high thermal oxidative stability (TOS), low moisture regain, and low conversion costs. While polyimide polymers exhibiting one or some of these qualities are known, there is a need for polyimide polymers that have all of these advantages.
Known composite systems having desirable characteristics include systems designated as Avimid.RTM. K and Avimid.RTM. N (E.I. du Pont de Nemours and Co.).
Avimid.RTM. K, described in A. R. Wedgewood, SAMPE Tech. Conf. 24, p. T385, 1992, employs a matrix based on pyromellitic dianhydride and extended ether diamines. These polymers are compatible with composite molding processes featuring melt consolidation of devolatilized prepreg plies. The glass transition temperature range of the polyimide component of Avimid.RTM. K is in the range of from 220 to 255.degree. C.
Avimid.RTM. N utilizes a polyimide polymer prepared from 2,2'-bis(3,4-dicarboxyphenyl)-hexafluoropropane tetracarboxylic acid (6FTA), the dianhydride form of which is referred to as 6FDA, along with a mix of p-phenylenediamine (PPD) and m-phenylenediamine (MPD). This polyimide polymer, designated and known in the art as NR-150, has a glass transition temperature range of about equal to or greater than 340.degree. C., with a reported moisture regain of about 3.7% by weight. See R. J. Boyce and T. P. Gannett, HIGH TEMPLE WORKSHOP XV, N, January 1995.
Another known polyimide polymer is comprised of biphenyl dianhydride and optionally pyromellitic dianhydride (PMDA) with 1,4-bis(4-aminophenoxy)-2-phenylbenzene (2PhAPB144) and MPD or PPD. See U.S. Pat. No. 5,478,913. This polymer exhibits a glass transition temperature range of about 220 to about 330.degree. C., with a moisture regain reported in R. J. Boyce and T. P. Gannett, HIGH TEMPLE WORKSHOP XV, N, January 1995, of about 2.8%.
The current invention provides novel polyimides which have relatively high Tg and with thermal oxidative stability and moisture regain characteristics significantly better than those exhibited by 6FTA or 6FDA based resins. The polyimide polymers of the current invention have good mechanical properties, making them useful in a wide variety of applications. Moreover, the invention has the potential of cost savings over the polyimides of the state of the art because the acid functional components of the current invention are relatively less expensive than the acid functional components of known polyimide polymers.